isothermal precision forging of aluminum alloy ring seats with different preforms using fem and...

ORIGINAL ARTICLE

Isothermal precision forging of aluminum alloy ring seatswith different preforms using FEMand experimental investigation

Yanqiu Zhang & Shuyong Jiang & Yanan Zhao &

Debin Shan

Received: 13 August 2013 /Accepted: 13 March 2014 /Published online: 1 April 2014# Springer-Verlag London 2014

Abstract 7A09 aluminum alloy ring seat of airplane is sub-jected to isothermal precision forging. The influence of thedifferent preforms on flow line, microstructures, mechanicalproperties, and defects of the forging is comprehensivelyinvestigated by means of experiments and FEM. Isothermalprecision forging of the ring seat is implemented on the basisof three disk preforms with the height of 15, 25, and 35 mm,respectively. The experimental results indicate that the flow ofmetal along the radial direction increases with the increase inthe height of the preform, and large plastic deformation ofmetal along the radial direction contributes to forming flowline of the forging. In the case of the preformwith the height of35 mm, the forging exhibits perfect profile, where there existno defects such as underfilling and folding, while breaking offlow line frequently occurs. Furthermore, the high preformcauses the forging to possess finer grain and substructure andconsequently contributes to enhancing comprehensive me-chanical properties. As a consequence, the preforging preformis appropriately designed so that high-quality forging withperfect flow line can be obtained by means of the optimumprocess procedure.

Keywords Finite element method . Aluminum alloy . Bulkdeformation . Flow line . Isothermal forging

1 Introduction

Isothermal precision forging is an advanced plastic formingprocess, in which the dies are heated to the approximatelysame temperature as the forging and the temperature of theforging is almost constant during forging [1, 2]. As a candidatefor manufacturing a net-shaped or at least a near net-shapedworkpiece, isothermal precision forging has been increasinglyused for forming light materials such as aluminum alloy [3],magnesium alloy [4], titanium alloy [5], titanium aluminide[6], aluminum matrix composite [7], and magnesium matrixcomposite [8] which are characterized by a small forgingtemperature range. As compared to conventional non-isothermal forging [9, 10], isothermal precision forging hasmany advantages, such as uniform temperature distribution,low deformation load, high material plasticity, and smallmachining allowance. Accordingly, on the basis of the exper-iment, isothermal precision forging has been successfullyapplied to manufacturing the forgings with light weight andhigh property in the aerospace industry, such as cylindricalhousing [11], compressor blade [12], rotator [13], turbineblade [14], turbine disk [15], and engine blade [16]. However,from the perspective of the process design, isothermal preci-sion forging must meet the requirements for forging profile,dimension accuracy, flow line, and microstructural evolutionsince any defects of the forging may lead to the failure of theforging. Therefore, based on finite element method (FEM),the optimization of the process parameters plays an importantrole in obtaining high-quality forgings during isothermal pre-cision forging. In recent years, many researchers have devotedthemselves to investigating the optimization of isothermalprecision forging by means of FEM. Yang et al. appliedthree-dimensional rigid-viscoplastic FEM for simulating iso-thermal precision forging of the blade of aeronautical engine[17]. Petrov et al. investigated the formation mechanism of alap defect during isothermal forging of an aluminum alloy part

Y. Zhang : S. Jiang (*) :Y. ZhaoIndustrial Training Centre, Harbin Engineering University,Harbin 150001, Chinae-mail: [email protected]

D. ShanSchool of Materials Science and Engineering, Harbin Institute ofTechnology, Harbin 150001, China

Int J Adv Manuf Technol (2014) 72:1693–1703DOI 10.1007/s00170-014-5784-9

with a deep central cavity and irregular shape by means ofFEM [18]. Rao et al. applied FEM for obtaining the localstrain variation during isothermal forging of magnesium alloypart so as to analyze the anisotropy of metal flow [19]. Zhanget al. evaluated shear friction factor of TA15 titanium alloyunder isothermal forming process by combining experimentswith FEM [20]. Shan et al. established a three-dimensionalrigid-plastic finite element model to simulate isothermal pre-cision forging of magnesium alloy bracket, which lays thefoundation for optimizing the bracket preform [21]. Luri et al.combined two-dimensional FEM with three-dimensionalFEM in order to analyze the isothermal forging of a nano-structured connecting rod, in which two-dimensional FEM isused to determine the geometric parameters of the cross sec-tion, while three-dimensional FEM is used to determine theother geometric parameters [22].

It is well known that aluminum alloy is most widely used inaircraft due to light weight as well as low cost. In general, thecomplex-shaped forgings of aluminum alloy possess the light-ening structures, such as high rib, long ear, thin web, and thinwall. Furthermore, aluminum alloy usually has a narrow forg-ing temperature interval of about 70 °C, and thus, it is espe-cially suitable for isothermal precision forging [23]. In theprevious study, isothermal precision forging was successfullyused for manufacturing complex-shaped rotating disk of alu-minum alloy on the basis of processing map and digitizedtechnology [24]. In addition, the flow line of complex-shapeddisk forging was systematically investigated through experi-ment and FEM, where the experimental results are in goodaccordance with the simulated ones [25]. In the present study,7A09 aluminum alloy ring seat of airplane was subjected toisothermal precision forging and the influence of the differentpreforms on flow line, microstructures, mechanical properties,

and defects of the forging was comprehensively investigatedby means of experiments and FEM, which has never beenreported in the literatures so far.

2 Experimental conditions

2.1 Ring seat forging

Ring seat of aluminum alloy is an important load-bearing part,and it is located in the principal shaft of the lifting system ofairplane. Furthermore, the forging of the ring seat belongs toan asymmetrical disk-like structure with thin wall, thin web,high rib, and long ears, as shown in Fig. 1. In addition, theforging of the ring seat must meet the requirement for thedistribution of flow line along the profile of the forging inorder to guarantee the appropriate load-bearing ability, asshown in zone A and zone B of Fig. 1c. In the present work,from the perspective of low cost and high efficiency, theforging is reduced to one third of the genuine forging ofairplane in scale in order to better understand the metal flow.

2.2 Materials

7A09 aluminum alloy bar with a diameter of 200 mm is usedas the experimental material, and it is manufactured by meansof extrusion. The chemical composition of 7A09 aluminumalloy is shown in Table 1.

2.3 Isothermal precision forging experiment

For the purpose of understanding the flow law ofmetal as wellas obtaining the optimum working conditions, isothermal

Fig. 1 Model of ring seatforging: a forward face, bbackward face, and c flow linedistribution

1694 Int J Adv Manuf Technol (2014) 72:1693–1703

precision forging of the ring seat was carried out on thehydraulic press of 50,000 kN based on three kinds of billetswith the same volume and the different dimensions. The threebillets possess the diameter of 155 mm and the height of30 mm, the diameter of 120 mm and the height of 50 mm,and the diameter of 100 mm and the height of 70 mm, respec-tively. Furthermore, they were all derived from the original7A09 aluminum alloy bar. The isothermal forging is dividedinto two stages, namely, upsetting and die forging. In the caseof upsetting, the three billets were heated to 430 °C and wereheld for a certain time. Subsequently, they were deformed by50 % and were made into the disk preforms with the height of15, 25, and 35 mm, respectively. In the case of die forging, thethree disk preforms were heated to 100 °C in the heatingfurnace and were covered with colloidal graphite mixed withwater. Then, the disk preforms were reheated to 430 °C andwere held for 1.5 h. Simultaneously, the dies were directlyheated to 100 °C on the hydraulic press and were covered withcolloidal graphite mixed with water. Subsequently, the dieswere reheated to 430 °C. Finally, the disk preforms were putinto the dies and were subjected to isothermal precisionforging.

2.4 Observation of flow line

The forgings of the ring seat were cut along the cross sectionas shown in Fig. 1c, and subsequently, they were subjectedto coarse grinding, fine grinding, and mechanicalpolishing. All the specimens polished were etched in asolution containing 10 % NaOH and 90 % H2O by volumefraction, and then, they were put into a solution containing30 % H3NO3 and 70 % H2O by volume fraction. The

observation of flow line was performed by means of ascanning instrument.

2.5 Heat treatment

The forgings of the ring seat were subjected to T73 whichstands for solution treatment and overaging treatment. Theprocedure of T73 is described as follows. Firstly, the forgingswere heated to 465 °C and were held for 45 min. Subsequent-ly, they were quenched into water. Then, the quenched forg-ings were heated to 110 °C and were held for 8 h. Finally, theywere heated to 177 °C and were held for 10 h.

2.6 Test of properties

Tensile test, hardness test, and electrical conductivity test werecarried out for the samples subjected to heat treatment, wherethe sampling location is shown in Fig. 1c. In the present study,electrical conductivity test is used to replace stress corrosiontest since stress corrosion resistance of metal material keeps apositive correlation with electrical conductivity [23]. Tensiletest was performed on a universal tensile testing machine ofInstron5569. Hardness test was carried out on a Brinell hard-ness machine of HB3000B. Electric resistance of the sampleswas measured by means of Source Meter2400, and electricalconductivity was further calculated by combining Eq. (1) withEq. (2).

R ¼ ρ� LA¼ 1

σ� LA

ð1Þ

σ ¼ 1

R� LA

ð2Þ

where R is the electric resistance, ρ the electric resistivity, σthe electrical conductivity, L the length of the sample, and Athe cross section of the sample.

Table 1 Chemical composition of 7A09 aluminum alloy (%, massfraction)

Cr Mn Si Cu Zn Mg Ti Fe Al

0.23 0.081 0.063 1.49 5.8 2.8 0.024 0.45 Balance

Fig. 2 Finite element model based on the different preform heights: a h=15 mm, b h=25 mm, and c h=35 mm

Int J Adv Manuf Technol (2014) 72:1693–1703 1695

2.7 Microstructural observation

Microstructures of 7A09 aluminum alloy samples werecharacterized by electron backscatter diffraction (EBSD)

using a S-4700 scanning electron microscope (SEM)and transmission electron microscopy (TEM) usingPhilipsCM12. All the samples were derived from theforging subjected to heat treatment. The samples for

Fig. 3 Velocity fields of ring seat forging based on the different preform heights by finite element simulation: a h=15mm, b h=25mm, and c h=35mm


Fig. 4 Formability of ring seatforging based on the differentpreform heights: a h=15 mm, bh=25 mm, and c h=35 mm


EBSD observation were made with the length of10 mm, width of 10 mm, and height of 2 mm. Subse-quently, they were electropolished in an electrolyteconsisting of 90 % C2H5OH and 10 % HClO4 byvolume fraction. Foils for TEM observation were me-chanically ground to 100 μm, and then, they werethinned by twin-jet polishing in an electrolyte consistingof 40 % CH3OH, 50 % HNO3, and 10 % H2O byvolume fraction. Fractographs for the tensile sampleswere examined by means of SEM using CambridgeS240.

3 Finite element simulation condition

3.1 Material model

According to the compressive stress–strain curves of 7A09aluminum alloy at the strain rates ranging from 0.01 to 10 s−1

and at the temperatures ranging from 300 to 460 °C, the

corresponding constitutive equation is established asfollows [24]:

ε� ¼ 1:48� 107 sinh 0:0124σð Þ½ �4:906 � exp −101:3� 103.RT

� �

ð3Þ

The constitutive equation shall be used as the materialmodel during finite element simulation of isothermal forgingof the ring seat.

3.2 Finite element model

In our previous work, based on commercial DEFORM3Dfinite element code, finite element model has been success-fully used to simulate isothermal precision forging ofcomplex-shape rotating disk of aluminum alloy, where theexperimental results have been in good agreement with thesimulation ones [24]. In the present study, a three-dimensionalmodel of finish forging die for isothermal precision forging isfirstly established according to a three-dimensional model of

Fig. 5 Flow line of ring seatforging based on the differentpreform heights: a h=15 mm, bh=25 mm, and c h=35 mm


the forging of the ring seat as shown in Fig. 1, and then, three-dimensional models of three disk preforms are establishedaccording to the genuine size in the process experiment. Thedie model and the preform model are transformed intoDEFORM3D finite element code through the interface be-tween computer-aided design (CAD) and computer-aided en-gineering (CAE). Consequently, finite element models areobtained according to three disk preforms, as shown inFig. 2, where the preforms are defaulted as deformation bodybased on rigid-viscoplastic material, and the dies are defaultedas rigid body. Furthermore, three disk preforms are all dividedinto 50,284 elements. In DEFORM3D software, the preformis defined as slave and the dies are defined as master. Inaddition, the parameters for finite element simulation aredetermined as follows. Firstly, the movement velocity of topdie is defaulted as 1 mm/s. Secondly, the frictional model isbased on shear model and the frictional coefficient is selectedas 0.3. Thirdly, the temperatures of the dies and the preformsare defaulted as 430 °C.

4 Results and discussion

4.1 Influence of preform height on metal flow

Figures 3 and 4 indicate a comparison of finite elementsimulation results with experimental ones during isothermalforging of the ring seat. It can be seen from simulation resultsin Fig. 3 that the metal firstly flows along the axial directiondue to the minimum resistance. When the die impression isfull of metal in the axial direction, the redundant metal

continues to flow into the die impression along the radialdirection. When the preform is 15 mm high, no sufficientmetal flows into the die and consequently, the forging exhibitsthe defect of underfilling, as shown in Fig. 4a. When theheight of the preform is 25 mm, the die impression is filledwith metal but the folding defect occurs in the forging, asshown in Fig. 4b. However, when the preform is 35 mm high,the die impression is completely filled and the correspondingforging exhibits no defects, as shown in Fig. 4c. Therefore, itcan be concluded that the preform height has a significantinfluence on metal flow during isothermal forging of the ringseat. Furthermore, it is very necessary to control appropriatemetal flow along the axial and radial directions, which playsan important role in forming the high-quality forging.

4.2 Influence of preform height on flow line

Figure 5 indicates the distribution of flow line of the forgingsin the case of the preforms with the height of 15, 25, and35 mm, respectively. It can be found from Fig. 5 that the flowline of the forging becomes more and more predominant withthe increase in the height of the preform. In particular, in thecase of the preform with the height of 15 mm, the forgingshows unclear flow line, while on the basis of the preformwith the height of 35 mm, the flow line of the forging is veryapparent. The phenomenon indicates that flow line is closelyrelated to the radial metal flow. In general, large plastic defor-mation of metal along the radial direction contributes toforming the flow line of the forging. Furthermore, it can beobserved from Fig. 5b, c that the flow line of the forgingshows the defects, such as disorder of flow line and breaking

Fig. 7 EBSD maps ofmicrostructure of ring seat forgingbased on the different preformheights showing the substructuresin the grain: a h=15 mm, b h=25 mm, and c h=35 mm

Fig. 6 EBSD maps ofmicrostructure of ring seat forgingbased on the different preformheights showing the grain shape:a h=15 mm, b h=25 mm, and ch=35 mm


of flow line. The disorder of flow line is attributed to inhomo-geneous plastic deformation of metal along the radial direc-tion. However, the breaking of flow line is attributed to the toothin web plate of the forging. When the web plate of theforging is very thin and the preform is very thick, large plasticdeformation of metal along the axial direction leads to soexcessive metal flow along the radial direction that the flowline formed is completely broken by the following metal.

4.3 Influence of preform height on microstructures

Figure 6 indicates EBSD maps of microstructures of theforgings based on three disk preforms. It can be seen fromFig. 6 that as for the preforms with the height of 15 and25 mm, the forgings exhibit a larger grain size, while in thecase of the preform with the height of 35 mm, the forgingshows a smaller grain size and a larger grain elongation. Thephenomenon indicates that the height of the preform has animportant influence on the grain size of the forgings. Ingeneral, the grain size decreases with the increase in the heightof the preform since the higher preform leads to larger plasticdeformation. In addition, it can be found from the EBSDmapsin Fig. 7 that plenty of fine equiaxed grains distribute at thegrain boundary and there are plenty of substructures of dislo-cation cell in the grain interior. The phenomenon is attributedto the occurrence of dynamic recovery and dynamic recrys-tallization during isothermal forging, and it seems that dynam-ic recovery is dominant. Furthermore, the substructures arerefined more and more considerably with the increase in theheight of the preform. The refined substructures contribute toimproving the mechanical properties of the forging. It can beobserved from TEM photographs in Fig. 8 that there exists agreat deal of precipitation phase in the grain interior and at thegrain boundary. The precipitation phase can play an importantrole in enhancing the strength of the forging by means ofdispersion strengthening.

4.4 Influence of preform height on properties

Table 2 indicates the properties of the forgings based on threedisk preforms. It can be seen from Table 2 that as for the

preform with the height of 15 mm, the corresponding forgingpossesses the poorest mechanical properties, while in the caseof the other two preforms, the corresponding forgings exhibitslight variation in terms of yield strength and tensile strength.It seems that the preform with the height of 15 mm causes theforging to possess the maximum electrical conductivity, whichmeans that large plastic deformation leads to poor stresscorrosion resistance of the forging. However, in the case ofthe preform with the height of 35 mm, the elongation of theforging is the maximum and thus, the forging possesses higherplasticity, which can be further validated by tensilefractographs as shown in Fig. 9. Figure 9a, b indicates thatthe tensile fractographs exhibit no obvious dimples, whileFig. 9c shows that the tensile fractograph exhibits large anddeep dimples, which means that the forging is subjected toductile fracture.

4.5 Optimum isothermal precision forging

It can be found from the above analysis that the disk preformis unable to guarantee the high-quality forging with perfectflow line since the profile of the forging is very complicated,so it is necessary to design the appropriate profile of thepreforging preform. At the stage of upsetting of the originalaluminum alloy billet, the ratio of height to diameter should beincreased so as to lead to the radial distribution of flow line byincreasing the deformation degree in the axial direction of thebillet. Consequently, in the case of subsequent die forging,metal flow can be decreased along the radial direction in orderto avoid the occurrence of flow line disorder as well as flowline breaking. Furthermore, the web of the forging must meet

Table 2 Properties of ring seat forging based on the different preformheight

Preformheight (mm)

Tensilestrengthσb (MPa)

Yieldstrengthσs (MPa)

Elongationδ (%)

HardnessHBS

ElectricalconductivityICAS (%)

15 432.4 331.3 5.5 125 24.79

25 500.3 387.7 9.2 144.6 23.46

35 509.2 386.3 13.6 142.2 23.76

Fig. 8 TEM photographs ofmicrostructure of ring seat forgingbased on the different preformheights: a h=15 mm, b h=25 mm, and c h=35 mm


the requirement for thin structure, as shown in Fig. 1. Accord-ingly, the thin structure should be designed in the correspond-ing zone of the preforging preform in order to prevent theoccurrence of flow line breaking, which is attributed to the factthat plenty of metal flows outside along the radial directionduring finish forging. In addition, more metal should be usedfor forming four ears of the forging, so sufficient metal must

be given in the corresponding zones of the preforging pre-form. Simultaneously, the thin structure must be proposed inthe corresponding zone of the preforging preform in order toavoid the occurrence of the coarse grain due to severe sheardeformation in the outer wall of the ring seat. Therefore, forthe purpose of obtaining the qualified forging, the processprocedure is optimized as follows. Firstly, the original 7A09

Fig. 11 The process ofisothermal precision forging ofring seat: a preforging, b finishforging, and c pickled finishforging with flash removed

Fig. 9 Fractographs for thetensile samples derived from ringseat forging based on the preformheight: a h=15 mm, b h=25 mm,and c h=35 mm

Fig. 10 Dies used for formingring seat forging: a preforging die,b finish forging die


aluminum alloy bar with the diameter of 70mm and the heightof 150 mm is deformed to the disk preform with the height of30 mm by upsetting. Secondly, the disk preform is heated to100 °C in the heating furnace and then, it is covered withcolloidal graphite mixed with water. Subsequently, the diskpreform is reheated to 430 °C and is held for 1.5 h. Simulta-neously, the preforging die as shown in Fig. 10a is directlyheated to 100 °C on the hydraulic press and then, it is coveredwith colloidal graphite mixed with water. Subsequently, thepreforging die is reheated to 430 °C and the disk preform issubjected to preforging by means of preforging die. Thirdly,the preforging preform as shown in Fig. 11a is heated to430 °C in the heating furnace and is held for 1.5 h. Simulta-neously, the finish forging die as shown in Fig. 10b is directlyheated to 100 °C on the hydraulic press and then, it is coveredwith colloidal graphite mixed with water. Subsequently, thefinish forging die is directly heated to 430 °C and thepreforging preform is subjected to finish forging by meansof finish forging die, as shown in Fig. 11b. The final forging ofthe ring seat is obtained by removing flash as well as pickling,as shown in Fig. 11c. In addition, the perfect flow line isformed in the final forging, where no defects of flow linedisorder and flow line breaking occur, as shown in Fig. 12.

5 Conclusions

Isothermal precision forging is used for obtaining 7A09 alu-minum alloy ring seat of airplane by means of experimentsand FEM. The shape and the size of the preform are closelyrelated to the radial flow of metal and thus have an importantinfluence on flow line, microstructures, mechanical proper-ties, and defects of the forging. Three disk preforms with theheight of 15, 25, and 35 mm, respectively, are used aspreforging preforms during isothermal precision forging ofthe ring seat. In the case of the preform with the height of15 mm, the forging exhibits the underfilling defect as well asthe unclear flow line. In the case of the preformwith the heightof 25mm, the forging exhibits the folding defect as well as thedisorder of flow line. In the case of the preformwith the heightof 35 mm, the forging exhibits perfect profile, where there are

no defects such as underfilling and folding, while breaking offlow line frequently occurs. Furthermore, the 35-mm-highpreform leads to a finer grain and substructure of the forgingand consequently, it contributes to enhancing the comprehen-sive mechanical properties, such as high yield strength, hightensile strength, and high elongation. Therefore, the appropri-ate design of preforging preform as well as the proper controlof metal flow plays a significant role in obtaining the high-quality forging. Consequently, the qualified forging with per-fect flow line can be obtained by means of appropriatepreforging preform and optimum process procedure.

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isothermal precision forging of aluminum alloy ring seats with different preforms using fem and...

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